1. Field of the Invention
This invention relates to inhibiting stress cracking in stainless steel. More particularly, this invention relates to the use of metallic mercury to inhibit stress cracking of stainless steel in a chloride-ion environment. This invention further contemplates mercury/stainless steel as an amalgam resistant to stress cracking in a chloride-ion fluid environment.
Stainless steel, particularly austenitic chromium-nickel-stainless steels, has found widespread use in many applications such as boilers, power plants and the like. For instance, stainless steel is widely used in such areas as the petrochemical field, desalinization installations and electric utility plants. From its inception, the use of stainless steel has always presented some problems in its industrial applications; but, in particular, there is the pesky unpredictable problem of what is referred to as "stress corrosion cracking", that is, the occurrence of brittle fractures in steel.
While the term stress corrosion cracking is widely used to describe such stainless steel fractures, it is thought that the inclusion of the word "corrosion" in such a term is misleading. While undoubtedly cracking in stainless steel is a corroding process, corrosion of the type that causes cracking is focused on those specific areas of the stainless steel surface at which cracking occurs rather than throughout the stainless steel surface as the term corrosion implies. Therefore, it is thought that a more appropriate term for such cracking is "stress cracking" and this term is used hereinafter to describe the occurrence of such fractures in stainless steel.
Because the risk of industrial failures of austenitic stainless steel by stress cracking in chloride-bearing electrolytes is high, considerable effect has been made into eliminating or substantially reducing such cracking since not only is replacement of the stainless steel expensive, but also the cost of plant operations is increased from shutdown for repairs. Stress cracking in stainless steel results from the simultaneous action of tensile stress and a chloride-oxygen containing environment. The chloride-bearing electrolytes are recognized as being a source of cracking of austenitic stainless steels; and although there is not complete agreement on the mechanism by which the chloride-ion attacks the steel, it is recognized as being the culprit.
2. The Prior Art
The potency of the chloride-ion in producing stress corrosion cracking has been well demonstrated. The authorities are in general agreement that the cracking can occur readily in stainless steel specimens immersed in water containing only a few parts per million of chloride-ions. The temperature of the chloride-ion containing water is not particularly significant in producing such cracking as, in addition to producing cracking at boiling temperatures and above, water temperatures as low as 75.degree. C. even in such dilute solutions have been known to produce cracking in stainless steel. Furthermore, such cracking appears to occur in specimens both stressed and unstressed; and there appears to be little difference between the resistance of stainless steel to cracking between the various types of stainless steel.
One prior art effort has been to attempt to eliminate the chloride-ions from the water or other solutions with which the stainless steel is contacted to thereby reduce stress cracking. In the nuclear reactor field, particularly in the power generating nuclear reactor plant, conduits such as pipes, tubing, pipe fitting, etc. are generally formed from stainless steels; and in such reactors, wherein both a primary and a cooling water system is utilized, it is extremely important to eliminate any cracking in the conduits of the cooling system since repair or replacement is difficult or, at best, extremely costly from the standpoint of down-time if stress cracking occurs. It is, therefore, the common practice to subject the water used in such cooling water systems to extensive purification processes to eliminate all traces of the chloride-ion to avoid such cracking. It can be understood that due to the vast quantities of water used in such systems such as a nuclear reactor cooling water system, such water purification processes are extremely expensive and utilize equipment of extremely high cost. Furthermore, even though the processed water is subjected to such purification processes, some traces of the chloride-ion generally remain so that stress cracking cannot normally be eliminated.